Transparent plastic is lighter and has better processability as compared to glass, and therefore exploited for optical materials such as a lens. However, since plastic has commonly a low refractive index, the lens becomes thicker. This results in such problems as loss of lightness in weight and low heat resistance.
For improving a refractive index of plastic, there is a method of introducing a sulfur atom in a molecular structure of plastic. Since a compound having a thiol group can be thermocured with epoxies and isocyanates, a sulfur atom originated from a thiol group can be introduced in the structure of plastic. A cured product thus obtained has a high refractive index. Particularly, a thiol urethane resin (for example, see Japanese Unexamined Patent Publication No. 3-236386) obtained by reaction with isocyanates is exploited for a lens and other purposes. However, it has not been satisfactory in terms of heat resistance.
On the other hand, a compound having a thiol group can be photocured with a compound having a carbon-carbon double bond by an ene-thiol reaction. The advantages of the ene-thiol reaction are that the reaction proceeds by irradiation of ultraviolet rays regardless of with or without a polymerization initiator, that the reaction is not susceptible to reaction inhibition by oxygen and that the curing contraction is low, as compared to radical polymerization, which is a common operation in photocuring. With regard to a curing method and a cured product exploiting this reaction, a method using an unsaturated thiol compound having a carbon-carbon double bond and a thiol group in a molecule (for example, see Japanese Unexamined Patent Publication No. 49-51333), and a resin composition comprising a compound having a plurality of carbon-carbon double bonds and a compound having a plurality of thiol groups in a molecule (for example, see Japanese Unexamined Patent Publication No. 49-54491, Japanese Unexamined Patent Publication No. 50-27836, Japanese Unexamined Patent Publication No. 53-13409 and Japanese Unexamined Patent Publication No. 2003-295431) are proposed. Thus, the ene-thiol reaction enables to produce a cured product of thick film, so that it becomes possible to produce an article with thickness such as a lens. However, the resultant cured product was also not satisfactory in terms of heat resistance.
In recent years, so-called an organic-inorganic hybrid technology is drawing attention as a means of further enhancing the properties of an organic material, in which an inorganic material is composited to an organic material so as to provide properties of the inorganic material such as high heat resistance, chemical resistance and surface hardness. Among the technology, a method excelling in transparency and enabling thick film curing is the organic-inorganic hybrid method exploiting silsesquioxane. Silsesquioxane is a kind of silica and represented by RSiO3/2. Since it easily provides an organic-inorganic hybrid cured product by bringing a substituent that has reactivity with an organic material into the R position, the practical utilization thereof has been studied (for example, see Japanese Patent No. 3653976, Japanese Patent No. 3598749, Japanese Unexamined Patent Publication No. 10-330485 and Published Japanese Translation No. 2003-533553 of the PCT Application). Although these organic-inorganic hybrid cured products excel in heat resistance, there exists a problem that the refractive index is low in general, since the inorganic component is silica whose refractive index is low.
With regard to an organic-inorganic hybrid that introduces a sulfur atom for the purpose of increasing a refractive index, a composition comprising silicone having a carbon-carbon double bond and silicone having a thiol group (for example, see Japanese Unexamined Patent Publication No. 56-110731, Japanese Unexamined Patent Publication No. 60-110752, Japanese Unexamined Patent Publication No. 05-320511 and United States Patent Application Serial No. 2004/209972) is known. However, satisfactory heat resistance and surface hardness are not obtained by the methods provided in the descriptions in these patent documents, since the inorganic component to be used is silicone (rubber state at room temperature).